Method and apparatus for restoring a network

ABSTRACT

A method and apparatus for restoring communications in a network. The network includes a plurality of nodes, with each pair of nodes connected by a link, and with each link having information channels and restoration channels. An idle signal is sent on each restoration channel for each link. The failure of a link is detected, with the failed link connecting an originating node with a terminating node. In addition, the failed link includes at least one information channel carrying information signals. An alternate path through the network is determined for the information signals using restoration and idle signals sent over the restoration channels. The information signals are then routed from the originating node to the terminating node in accordance with the alternate path.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/629,458, filed on 29 Jul. 2003, now U.S. Pat. No. 7,142,505, which isa continuation of U.S. patent application Ser. No. 09/477,595 filed Jan.4, 2000 that issued as U.S. Pat. No. 6,600,719 on 29 Jul. 2003, whichclaims the benefit of U.S. patent application Ser. No. 08/936,369 thatissued as U.S. Pat. No. 6,130,876 on 10 Oct. 2000.

FIELD OF THE INVENTION

The invention relates to network restoration techniques in general. Moreparticularly, the invention relates to a method and apparatus forrestoring network communications when a node or link of the networkfails.

BACKGROUND OF THE INVENTION

It is a fairly accepted truism that communications is the lifeblood ofbusiness. As domestic and international businesses continue to expand atan extrodinary pace, these businesses become increasingly reliant upontelecommunications services to remain competitive in a globalmarketplace. Whether it is talking to a customer over the PublicSwitched Telephone Network (PSTN), sending an electronic mail messageover the Internet, or trading product specifications over a local areanetwork, disruptions to a communications network can mean significantlosses to a business. Extended outages are particularly harmful, buteven brief outages can be bothersome. The result is ever increasingdemands by telecommunications customers for a virtually uninterruptiblenetwork.

One element to creating a virtually uninterruptible network is tocorrect network outages as rapidly as they occur. At a very high level,a network can be viewed as a pattern of communications nodesinterconnected by communications links. The communications nodes caninclude electronic or optical cross-connects (“switches”), personalcomputers, servers, printers, or any other type of network device. Thecommunications links-include some type of media for transportingcommunications signals, such as optical fiber, twisted-pair copperwires, co-axial cable, radio frequencies, and so forth. An example of acommunications network would be a set of communications switches(“switching fabric”) connected together by optical fibers (“opticallinks”). If an optical link is damaged, as frequently occurs such aswhen a construction company digs in the area where the optical link isburied, the communications signals carried by the optical link must bequickly re-routed. This is also true if a switching fabric becomesinoperable, although the problem of re-routing the communicationssignals becomes an even greater challenge in this case.

Several conventional techniques have been developed to restorecommunications in the event of a link or node failure on-a network.These techniques are loosely referred to as “network restorationtechniques,” and in most cases refer to an algorithm for re-routing thecommunications signals carried by the failed link, or switched by theinoperative node. For example, a class of algorithms have been developedthat are referred to as “flooding algorithms.” Communication messagesfor service restoration in case of a failure in the network aretransmitted through links between the switches. The switches thenelectronically process these messages to take appropriate action torestore the failed traffic in the event of, for example, a link failure.

There are basically two types of flooding algorithms for restoring thefailed traffic in the event of a link failure. The first is referred toas “link based restoration,” while the second is referred to as“path-based restoration. Path based restoration attempts to re-routefailed circuits between the originating node and destination node of theindividual circuits in the failed link. By way of contrast, link basedrestoration attempts to re-route all traffic around the failed linkregardless of the origination and destination of the bearer traffic onthe failed link.

Link based restoration and path based restoration each have theiradvantages and disadvantages. For example, link based restoration istypically faster than path based restoration, but is less efficient interms of restoration capacity utilization. Conversely, path basedrestoration is slower than link based restoration, but utilizesrestoration resources more efficiently since the origination anddestination nodes of the failed nodes are typically distributedthroughout the system.

These techniques, however, are unsatisfactory for a number of reasons.For example, a completely optical layer network above the SynchronousOptical Network (SONET) layer is fast becoming a reality. The opticalnetwork is being driven both by the commercial availability of densewavelength division multiplex (DWDM) technology and the continuinggrowth of traffic. Current DWDM systems are offering sixteen or moreOC-48 channels on a pair of fibers. In the future it may grow to morethan one hundred wavelengths, and the channel capacity may increase toat least 10 Gigabytes per second (Gbps). When a substantial number oflinks are deployed in the network, it will be necessary to manage thenetwork at the optical layer. This management will require thecapability to restore the network in the optical layer. Networking andrestoration at the optical layer is highly desirable for opticalswitching systems. No signal will undergo optical to electricalconversion at these optical cross-connect systems. Therefore,restoration from a failure in the network will either requirecommunication and processing messages between the optical cross-connectsystems in the optical domain or an auxiliary optical channel which willundergo optical to electrical conversion and processing just formessaging. It is desirable to eliminate the need of an auxiliary channelfor the purpose of restoration. Even if it is required for otherpurposes, it is extremely important that the processing required at eachnode remains simple for implementing a fast restoration technique in anoptical network. Conventional network restoration techniques fail toaddress any of these concerns, and are not designed to perform networkrestoration in the optical domain.

In view of the foregoing, it can be appreciated that a substantial needsexists for a method and apparatus for providing fast restoration from alink or a node failure in a network, that solves the above-discussedproblems.

SUMMARY OF THE INVENTION

The present invention includes a method and apparatus for restoringcommunications in a network. The network includes a plurality of nodes,with each pair of nodes connected by a link, and with each link havinginformation channels and restoration channels. An idle signal is sent oneach restoration channel for each link. The failure of a link isdetected, with the failed link connecting an originating node with aterminating node. In addition, the failed link includes at least oneinformation channel carrying information signals. An alternate paththrough the network is determined for the information signals usingrestoration and idle signals sent over the restoration channels. Theinformation signals are then routed from the originating node to theterminating node in accordance with the alternate path.

With these and other advantages and features of the invention that willbecome hereinafter apparent, the nature of the invention may be moreclearly understood by reference to the following detailed description ofthe invention, the appended claims and to the several drawings attachedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a network suitable for practicing one embodiment ofthe present invention.

FIG. 2 illustrates an optical cross-connect system suitable forpracticing one embodiment of the invention.

FIG. 3 illustrates a network having a link failure in accordance withone embodiment of the invention.

FIG. 4 is a block flow diagram of the steps for restoring a network inthe event of a link failure in accordance with one embodiment of theinvention.

FIG. 5 illustrates a network having a node failure in accordance withone embodiment of the invention.

FIG. 6 is a block flow diagram of the steps for restoring a network inthe event of a node failure in accordance with one embodiment of theinvention.

FIG. 7 is a block flow diagram of the steps for restoring a network inthe event of a node failure in accordance with another embodiment of theinvention.

FIG. 8 illustrates a block diagram of a detecting circuit in accordancewith one embodiment of the invention.

DETAILED DESCRIPTION

The present invention includes a method and apparatus for opticalnetwork restoration. One embodiment of the invention is link based andcomprises an automatically computed shortest route restorationalgorithm. It does not require a sender and chooser node identificationor band width requirement messages. It works from both endssimultaneously and therefore, the restoration is fast for link failure.This embodiment of the invention is also applicable for a node failurein the network. It includes an in-band restoration algorithm which doesnot require any extra channel or any overhead (as in SONET) andtherefore is optimal for an optical network restoration. The in-bandmessaging and restoration method achieves fast restoration comparable toa SONET ring restoration and yet with less network restoration capacity.

Referring now in detail to the drawings wherein like parts aredesignated by like reference numerals throughout, there is illustratedin FIG. 1 a network suitable for practicing one embodiment of thepresent invention. FIG. 1 shows an optical network 8 comprising multiplenodes labeled 1-5 connected by DWM optical links. It is assumed thatoptical network 8 is OC-48 based, i.e., each operating channel in eachDWDM optical link is carrying an OC-48 signal. In this embodiment of theinvention, there is an optical cross-connect system in each node. It canbe appreciated, however, that electrical cross-connects can be used andstill fall within the scope of the invention. In such a case, each nodewould be equipped with electrical to optical (O/E) and E/O converters,and the appropriate signaling across said O/E and E/O converters willhave to be implemented in accordance with the principles describedherein.

FIG. 2 illustrates an optical cross-connect-system suitable forpracticing one embodiment of the invention. The cross-connect systemincludes an optical switch fabric 30, multiplexers/demultiplexers (MUX)32, a central processing unit (CPU) 34, and memory 36. Memory 36 furtherincludes a network restoration module (NRM) 38.

Optical switch fabric 30 performs switching functions by routing asignal from one of its input port to one of its output port. Thisrouting is accomplished regardless of the rate of the signal.

MUX 32 performs wavelength adaptation and multiplexing/demultiplexingfunctions. With wavelength adaptation and multiplexing, a large numberof OC-48 (or any other rate) signals can be transported over a singlefiber. Similarly, the reverse function, wavelength de-multiplexing andadaptation, are performed in the receive direction before the OC-48signals interface with the optical cross-connect system.

It is worthy to note that in this embodiment of the invention, thewavelength adaptation and multiplexing or de-multiplexing functions areshown outside the cross-connect system. It can be appreciated, however,that these functions can be implemented in the photonic cross-connectsystem itself and still fall within the scope of the invention.

Optical switch fabric 30 is connected to CPU 34. In this embodiment ofthe invention, CPU 34 is a dedicated processor for optical switch 30,but also may be an external processor. In any event, CPU 34 must havesufficient processing power to perform the functionality for NRM 38.

CPU 34 operates under the control of executed computer programinstructions that are stored in memory 36. In this embodiment of theinvention, NRM 38 is in the form of computer program instructions storedin memory 36. It can be appreciated, however, that the functionality forNRM 38 may be implemented in hardware, software, or a combination ofhardware and software, using well-known signal processing techniques.

Memory 36 may be any type of machine readable storage device, such asrandom access memory (RAM), read only memory (ROM), programmable readonly memory (PROM), erasable programmable read only memory (EPROM),electronically erasable programmable read only memory (EEPROM), magneticstorage media (i.e., a magnetic disk), or optical storage media (i.e., aCD-ROM). Further, the cross-connect system may contain variouscombinations of machine readable storage devices accessible by CPU 38,and which are capable of storing a combination of computer programinstructions and data.

Referring again to FIG. 1, in optical network 8 each pair of nodes isconnected by one or more optical links, each link having a pair offibers for two-way transmission. Although each link is a two-way link,it is not necessary to assume that the traffic is also two-way for therestoration to work.

Optical network 8 is designed in such a way that a majority of thechannels in each link carry live traffic and a small number of channelsare left vacant to be used for restoration in case of a network failure.To meet a given objective for restoration against failures, a networkmust be designed properly with respect to the topology and per linkusage ratio. Issues associated with the design of such a network hasbeen studied extensively for electronic cross-connect based networks.The same principles can be applied to design an optical network withdesired restoration properties in the optical domain.

This embodiment of the invention utilizes three different types ofsignals that can be transmitted on the optical channels: (1) a servicetraffic signal (TS); (2) an idle signal (IS); and (3) a restorationsignal (RS). The IS is transmitted over the channels not carryingservice and are reserved for restoration in case of a failure in thenetwork. The RS is transmitted over the restoration channels when theyare to be used for service restoration in case of a failure in thenetwork.

The IS and RS may be any bit pattern, but must be simple enough for thecross-connects to detect, process and insert quickly. In this embodimentof the invention, a repeating sequence of “101010 . . . ” bit pattern isused as the IS. The RS is a repeated bit sequence indicating theoriginating node number associated with a failed link. For other typesof restoration, however, a different RS may be necessary. In any event,for any type of restoration, both IS and RS are of an arbitrarily lowbit rate and independent of the traffic bit rate. Therefore thecapability of timing recovery and detection of IS and RS messages isimplemented in the optical domain without the need for optical toelectrical conversion, or in the electronic domain with simplecircuitry. For example, even though the service channels may carry OC-48signals which need to be restored on the idle channels, the restorationchannels can use a bit rate of only 10 Mb/s or lower for IS and RS.

It is worthy to note that although this embodiment of the invention isillustrated using an optical network, it can be appreciated that thisembodiment of the invention is equally applicable to a general networkregardless of the types of links and nodes in a network or the types oftraffic carried on the links. An advantageous embodiment of theinvention, however, provides the most benefit for optical networkrestoration.

NRM 32 provides network restoration in the event of a link failure or anode failure. If there is a link failure, it is assumed that there is asingle link failure, since the probability of another independent linkfailure in optical network 8 within the restoration period of the firstlink (sub second) is small. If there is a node failure, however, alllinks connected to the failed node fail simultaneously.

Since the probability of a node failure is much lower than that of alink failure, NRM 32 first assumes that any failure is a result of alink failure and attempts to restore accordingly. That would result infast restoration, if indeed it is a link failure. If the failure, on theother hand, is due to a node failure, it will take a longer time torestore the failed channels.

FIG. 3 illustrates a network having a link failure in accordance withone embodiment of the invention. As shown in FIG. 3, each node inoptical network 8 is assigned a unique number with a predeterminedmaximum.

For illustration purposes only, optical network 8 is assumed to havefive nodes, numbered 1 through 5, as shown in FIGS. 1 and 3. Each pairof nodes is connected by an optical link. Further, assume that each linkhas eight wavelength channels. The channels left vacant for restorationare shown on each link. These channels carry IS in normal condition.FIG. 3 shows a failure in link the link between nodes 1 and 5 (“link1-5”). Note that as used herein links will be designated by the nodesthey connect, with the direction of signals being designated asoriginating at the first node and terminating at the second node. Thusfor link 1-5, signals are originating from node 1 and terminating atnode 5. For link 5-1, signals are originating from node 5 andterminating at node 1. When link 1-5 fails, service is restoredfollowing the method described with reference to FIG. 4.

FIG. 4 is a block flow diagram of the steps for restoring a network inthe event of a link failure in accordance with one embodiment of theinvention. At step 40, NRM 32 performs initiation for the originatingnode. Upon detection of failure of one or more channels in one of itslinks, a node inserts a signal carrying a binary word indicating its ownnode identification number (NID) to all outgoing healthy restorationchannels in all links. The node also sends the failure message to aNetwork Management System (NMS) (not shown).

It is worthy to note that in one embodiment of the invention NRM 32 runsas individual processes in each node, and therefore does not require acental controller. In another embodiment of the invention, NMS can beused as a cental controller to facilitate certain functions of NRM 32.In such a case, reference to a NMS will be made where appropriate.

Intermediate node connection is performed at step 42. When anintermediate node, that is not connected to the failed link, detects achange from IS to RS in an incoming restoration channel, it connects thechannel to one restoration channel in each outgoing link. If multiplechannels in an incoming link change from IS to RS (as in the case of alink failure) then the maximum number of such channels are connected tothe restoration channels in each outgoing link. Since there will be, ingeneral, more than one other link, these will be broadcast typeconnections.

At step 44, the loop release by the intermediate Node is performed.Subsequent to making connections according to step 42 in one direction,when an intermediate node receives another message signal from anotherlink, the node checks if the originating node number in this messagesignal is identical to that in the first message. If so, the link isreleased of all connections and IS is inserted in each restorationchannel on that link.

An intermediate node reverse connection is made at step 46. After anintermediate node has already made a connection in one direction, whenit receives a message signal from an idle channel of another of itsincoming links, it checks if the originating node number is differentfrom the one received during its first connection. If the originatingnode number is different, the intermediate node connects the maximumnumber of restoration channel carrying RS to the restoration channels inthe outgoing direction on the link whose incoming channels were firstconnected. It is worthy to note that in another embodiment of theinvention, the RS may carry both the originating and terminating nodeIDS. In that case each intermediate node matches both IDs in reverseorder for two directions of transmission.

The terminating node connection is performed at step 48. When a nodeassociated with the link failure receives messages carrying another nodeID on any incoming restoration channel, it disconnects the input portsfrom the outgoing channels on the failed link to these restorationchannels. Upon completion of the connections, the terminating nodeinserts IS in place of RS in to all other restoration channels. Theterminating nodes then inserts IS to the restored channels on the failedlink so that the receiving node is alerted when the link is repaired. Asa further confirmation, the terminating nodes may also check theoriginating node IDs in the incoming RS signals to ensure that they areindeed the nodes at the other ends of the failed link.

The intermediate node releases all connections at step 50. When thesignal in any restoration channel is changed from RS to IS, anintermediate node releases all connections to the incoming restorationchannel and inserts IS in the outgoing disconnected restorationchannels.

NRM 32 terminates network restoration at step 52. Steps 40 through 50are repeated until one of the following four conditions is satisfied:(1) all failed channels are restored; (2) there is no more restorationchannel on any outgoing link; (3) a predetermined time-out periodexpires and no RS is received in any incoming restoration channel; and(4) the node receives a command from the NMS to halt the process.

At step 54, all operations of optical network 8 are returned to normal.When the failed link is repaired the nodes associated with the failedlink receives IS from the repaired link. Then the restored signals areconnected to their original ports on the repaired link, the receivedsignals are checked by the receiving nodes for the validity of thesignals and then disconnected from the restoration channels. IS is theninserted on the disconnected restoration channels. Upon receiving theIS, the intermediate nodes remove the restoration connections andinserts IS to the outgoing restoration channels. The network thenreturns to its original state.

If the restoration process is terminated before all failed channels arerestored and yet spare channels are available, it is because either thefailure is only in the incoming direction (single-ended failure) or itis a node failure or there is no spare capacity between the two nodes.If it is because of single-ended failure, the NMS can detect that fromthe failure messages sent by the nodes. Then the NMS can command theinvolved nodes to restore the channel in both directions and the twonodes can then restore the channel following the above rules. Similarlyif it is due to node failure, the NMS can detect that from the failuremessages from the adjacent nodes and command the involved nodes to beginrestoration from a node failure which is discussed next.

The operation of NRM 32 can be better illustrated using the followingexample, which makes reference to FIG. 3. As shown in FIG. 3, link 1-5fails. Upon the failure of link 1-5, node 1 continuously transmits thebinary word “00000001 ” to all outgoing idle channels: channels 6, 7 and8 of link 1-3 and channels 7 and 8 of link 1-2. Specific implementationmethod of the coding and decoding of the optical message signal RS willbe discussed later. On detection of the failure of link 1-5, node 5 alsoperforms the similar functions as node 1 independently.

Upon detection of the change from IS to RS from channels on links 1-3and 1-2, nodes 2 and 3 read the originating NID (1 in this case).carriedon the restoration channels (6, 7 and 8 on link 1-3 and channels 7 and 8on link 1-2), and connect the incoming restoration channel ports to theoutgoing restoration channel ports in all other links. Since there aremultiple restoration channels in each link the channels may be connectedsequentially. Thus node 2 connects channels 7, 8 from link 1-2 tochannels 7, 8 of link 2-3 and also to channels 7 and 8 of link 2-4.Similarly node 3 connects channels 6 and 7 of link 1-3 to channels 7 and8 of link 3-4, to channels 7 and 8 of link 3-5 and to channels 7 and 8of link 3-2.

Within a short period of time after node 2 connects restoration channelsfrom link 1-2 to all other outgoing links, it will receive from node 3on link 3-2 RS with the NID as 1. Upon detecting that the restorationchannels 7 and 8 from link 3-2 have the same originating node number asin channels 7, 8 on link 1-2, node 2 releases all connections (channels7 and 8) to link 2-3 and changes the signal on these channels in link2-3 to IS. Similarly node 3 releases connections to channels 7 and 8 oflink 3-2 and inserts IS.

After node 3 has connected the restoration channels from link 1-3 tolink 3-5 and 3-4, at some point of time depending on the link lengths,it will receive either from node 4 or node 5 RS signals on therestoration channels. Assuming that the signal propagation time on link5-3 is shorter than that on links 5-4 and 4-3 combined, node 3 receivesRS from node 5 on the restoration channels 7 and 8 of link 5-3. Notingthat the originating node number is now 5, node 3 connects therestoration channels 7 and 8 to the outgoing channels 6 and 7 of link3-1. Note that node 3 does not connect channels 7 and 8 from link 5-3 tolink 3-2 because they are no longer connected in the 2-3 direction.

Upon receiving RS from restoration channels 6 and 7 from link 3-1, node1 disconnects the input ports from channels 1 and 2 on link 1-5 andconnects to channels 6 and 7 on link 1-3. Subsequently, node 1 insertsIS in channels 7 and 8 on link 1-2, in channel 8 of link 1-3 and inchannels 1, 2 in link 1-5. Node 5 performs similar functions after itreceives RS from link 3-5. Thus channels 1 and 2 on the failed link 1-5are restored on the path 1-3 (Channels 6 and 7) and 3-5 (Channels 7 and8).

Node 2 receives IS (changed from RS) in channels 7 and 8 on link 1-2.Node 2 then disconnects channels 7 and 8 on link 2-4 and inserts IS inthese channels. This process continues to all the intermediate nodes andall the remaining restoration channels become free.

Nodes 1 and 5 know that they still have to restore channels 3, 4 and 5on link 1-5. It waits for a predetermined period, say 1 millisecond, andthen follows steps 40 through 50 to restore channel 3 on route 1-3(using channel 8), 3-4 (using channel 7), and 4-5 (using channel 7). Inthe subsequent attempt, it restores channels 4 of link 1-5 on route 1-2(using channel 7), 2-4 (using channel 7), 4-5 (using channel 8). Thennode 1 attempts to follow steps 40 through 50 again for the remainingchannel 5. However, it never receives any RS from the incoming channel 8on link 2-1 because no more restoration channel is available. Node 1then terminates any further restoration attempt after a waiting periodexpires. Node 5 finds that there is no more outgoing restoration channelavailable in any link. It immediately terminates any further attempt forrestoration. Both nodes 1 and 5 communicate with the NMS that it couldnot restore channel 5 on link 1-5. The NMS attempts to restore servicecarried on channel 5 in link 1-5 at a lower layer such as SONET layer,Asynchronous Transfer Mode (ATM) layer or electronic cross-connectlayer. SONET layer restoration can be either ring restoration or pathbased restoration. An overall network restoration approach, however,must consider many important issues: network configuration and cost,coordination of restoration among various layers, speed of restoration,and prioritization of restoration of different services/channels.

Nodes 1 and 5 receive IS from channels on link 1-5 when it is repaired.On receiving IS, node 1 and node 5 bridge the corresponding input portsto the service channels 1 through 4 on link 1-5. Nodes 1 and 5 thencheck for the validity of the received signals in channels 1 to 4 andthen disconnect these signals from the restoration channels. It isworthy to note that channel 5 on link 1-5 was never disconnected becauseit was not restored in the optical layer. Nodes 1 and 5 then insert ISto outgoing restoration channels in links 1-2, 1-3 and 5 -3, 5-4. Theintermediate nodes 2, 3 and 4 disconnect the restoration channels in theoutgoing directions and insert IS.

NRM 32 provides the means of SONET equivalent span protection switchingin an optical line. If only one channel in a link fails, then NRM 32restores the channel on the restoration channel in the same linkprovided that is the shortest link between the two nodes. With respectto FIG. 3, if any one or more (up to three) channels on link 1-5 failthen they will be restored on channels 6, 7, and 8, provided 1-5 is theshortest path between nodes 1 and 5.

FIG. 5 illustrates a network having a node failure in accordance withone embodiment of the invention. If there is a node failure in thenetwork, the first attempt to restore a link as described previouslywill not be successful. When the time-out period is expired or a commandis received from the NMS, NRM 32 begins the restoration from a nodefailure. Note that unsuccessful link restoration attempt could have beendue to two or more simultaneous and independent link failures. Theprobability of another completely independent link failure within therestoration time of the first failed link, however, is extremely small.Simultaneous link failures, on the other hand, is most likely to beassociated with a node failure. Therefore, at the end of an unsuccessfullink restoration attempt, NRM 32 assumes that the cause of the failureis a node failure and it begins a node restoration process.Alternatively, NRM 32 can wait for commands from the NMS to begin noderestoration. The NMS, of course, can detect the node failure without anyambiguity. The steps performed for network restoration in the event of anode failure will be described with reference to FIG. 6.

FIG. 6 is a block flow diagram of the steps performed for restoring anetwork after a node failure in accordance with one embodiment of theinvention. Since a node failure causes multiple link failures, it is nownecessary to select and restore the failed channels sequentially toavoid congestion during restoration by flooding method. For thatpurpose, a connection map of the network is maintained in memory 36 ofeach cross-connect system. When a node determines that the cause of thefailure of its link to another node is the failure of the latter, itlooks up the connection map at step 60. From the connection map, eachassociated node determines its rank relative to the others. For example,the ranks can be assigned with higher to lower as the NID increases.

If the node is of highest rank, it begins restoration of the failedchannels on the link to the failed node at step 62. None of the othernodes connected to the failed node originates any restoration attempt.

To consider the restoration against node failure some aspects of thelink restoration method need to be modified. To restore a single linkfailure, it was sufficient that only the originating node ID wastransmitted in the RS signal. It is, however, desirable to include inthe RS signal both the originating and the target NIDs for confirmationpurposes. For restoration from node failure, on the other hand, it isnecessary that the RS signals carry both the originating and theterminating node IDs so that the optical paths can be restored aroundthe failed node.

The highest ranking node inserts RS to the restoration channels for alloutgoing links at step 64. The highest ranking node selects a failedchannel according to some order, which in this embodiment is on thebasis of priority. The node then inserts an RS signal, which containsthe originating NID (its own ID), terminating NID (the destination NIDfor the selected channel), to the first restoration channel in each ofthe outgoing links. If there are additional restoration channels in anyoutgoing links it selects the next failed channel and inserts an RS(with its NID and the selected channel's destination NID) to the nextrestoration channel in any outgoing link. The process is continued untilall failed channels or all outgoing restoration channels are exhausted.No attempt is made, however, to restore the failed channels which areterminated at the failed node. This can be accomplished by maintainingand consulting a table of terminating NIDs for each channel passingthrough the cross-connect system in its database.

There is at least one major difference between the single linkrestoration (SLR) and the single node restoration (SNR) functions of NRM32. In the case of SLR, each node works independently and simultaneouslyto restore the failed link. This method leads to a faster restoration.In the case of SNR, however, it is necessary to restore the channels ona failed link only from one node, namely, the higher ranking node toavoid congestion. When the receiving node (e.g., lower rank of the two)detects the RS signal from the higher ranking node, it restores thefailed channel whose destination is the higher ranking node at step 66.

After the highest ranking node either completes the restoration offailed channels connected to itself or all outgoing restoration channelsare exhausted at step 68, the next highest ranking node is selected atstep 70 and begins the restoration of the channels on its failed link.When the second ranking node decides that all failed channels terminatedat the highest ranking node are restored or a time-out period expires,it begins restoration of the channels that are terminated to all thelower ranking nodes. Note that none of the nodes will begin anyrestoration until all the channels that are terminated at a higherranking node are restored or a time-out period for the node expires.This process continues until all nodes restore their failed channels. Ifchannel restorations are not complete at step 68, steps 66 and 68 arerepeated until they are complete. Similarly, if node restorations arenot complete at step 72, steps 62 through 72 are repeated until they arealso complete.

Network restoration in the event of a node failure can be betterillustrated using the following example, which makes reference to FIG.5. Assume that highest to lowest ranking of the nodes are from 1 to 5.When node 3 fails, each node associated with the failed links attempt torestore the links. Of course, none will succeed because of no responsefrom node 3. After a time-out period, say, 100 milliseconds, theinvolved nodes 1, 2, 4 and 5 independently assume that the other end ofthe links, i.e., node 3 has failed. Node 1 being the highest rankingnode, begins restoration of the failed channels. Looking at thedestination nodes of the failed channels, it restores the channelsbetween itself and the destination nodes of the individual channels.Node 1, however, does not attempt to restore the channels whose finaldestination are node 3. In the meantime, nodes 2, 4 and 5 do not attemptto restore any channels yet. Each of these nodes, looking at the networkconnection map in its own database, determines which are the otheraffected nodes due to the failure of node 3. For example, node 2 knowsthat node 3 has failed and nodes 1, 4 and 5 besides itself are connectedto node 3. From this list it decides that its rank among the affectednodes is second after node 1. Therefore, it waits for a time-out period.This time-out period can be dependent on whether there is any failedchannel between nodes 1 and 2. If there is a channel between nodes 1 and2 through node 3 that failed, then node 2 waits until all the failedchannels between nodes 1 and 2 are restored or all restoration channelsare exhausted. On the other hand, if there is no failed channel betweennodes 1 and 2, then it can begin restoration of channels to other nodesimmediately. Node 2 begins restoration of any failed channels betweenitself and the affected lower ranking nodes 4 and 5. Nodes 4 and 5 stillwait for the node 2 to complete its restoration attempt in a similarmanner. This process is continued until the last but one node (node 4).Depending on the available restoration channels, some or all failedchannels will be eventually restored. Note that the restoration from anode failure will take longer time (of the order of 0.5 to 1 second)because simultaneous restoration attempts between multiple pairs ofnodes may interfere with each other leading to higher probability offailed attempts. The probability of congestion is minimized in theproposed method at the expense of longer restoration time for the lesslikely case of a node failure.

FIG. 7 is a block flow diagram of the steps performed for restoring anetwork in the event of a node failure in accordance with anotherembodiment of the invention. In this embodiment of the invention, steps62 through 72 are similar to the steps described with reference to FIG.6. In FIG. 7, however, the prioritization of nodes can be done by theNMS at step 74, rather than by the nodes themselves. In this case, theNMS will command the pairs of nodes to restore specific channelssequentially. The NMS can receive confirmation from the pairs of nodesbefore issuing another restoration command to avoid congestion in therestoration process.

NRM 32 is applicable for restoration of a failed link or a failed nodearound the nodes adjacent to the failure location. This is essentially alink based restoration. NRM 32, however, can perform path basedrestoration which provides more efficient utilization of restorationcapacity at the expense of restoration speed. The speed is compromisedfor several reasons: (1) the restoration cannot begin until the pathterminating node receives the path AIS (Alarm Indication Signal) fromthe nodes adjacent to the failure location; (2) more intermediate nodesare involved for a path restoration compared to link restoration; and(3) every pair of nodes for each failed path will simultaneously attemptto capture restoration capacity which may lead to conflict and itsresolution may take longer time. Considering the conflict resolutiondifficulties and restoration speed, link based restoration is consideredto be the more advantageous embodiment of the invention even though thepath based restoration is more capacity efficient. This determination,however, is contingent upon the type of network and desired performanceparameters, and is in this respect application specific.

The cross-connect systems must be able to detect the change of state ofeach of the incoming channels. As mentioned earlier, significantsimplification can be achieved by making the IS and the RS independentof the rate of the TSs. As an example, the IS can be a “101010 . . . ”pattern. The RS, on the other hand, will carry the originating node IDand the terminating node ID. Therefore, the signal can be generated as aframing pattern appended with two ID numbers of the originating andterminating nodes.

FIG. 8 illustrates a block diagram of a detecting circuit in accordancewith one embodiment of the invention. Each input port for the detectingcircuit has a splitter 84, a clock recovery circuit 80, and a decisioncircuit 82. The output for decision circuits 82 are sent to input portsfor optical switch fabric 30.

As shown in FIG. 8, each signal is fed to a clock recovery circuit 80and a decision circuit 82. The change in the state of the signal type isdecided by decision circuits 82 and a control signal is generated tocontrol the switching state of the switch fabric. The IS and RS bitrates should be identical so that the decision circuit can recognize thestate change from the IS to RS quickly. Since the TS and RS may be ofdifferent bit rates, the timing recovery circuit may take longer torecover clock and hence to recognize the state change from TS to IS.However, this state change recognition need not be fast.

The restoration time depends on the number of channels to be restored,number of restoration channels available in the alternate routes, thelink lengths, and to a less extent on the bit rate of the RS signal. Anestimate for the restoration time is given by the following equation:

$T_{r} = {{\frac{N\; 1}{c}\left( {N + 3} \right)} + {t_{p}{N\left( {N + 5} \right)}} + {Nt}_{w}}$where, N=Ceiling [f/(1-f)], the number of attempts required to restoreall channels in a failed link, f=Fraction of the channels in each linkused for normal traffic (1-f is the fraction of the channels in eachlink available for restoration), 1=Average link length, c=Speed of lightin fiber, t_(p)=Processing time at each node which includes the time todetect change in signal type and to set up or remove channelconnections, and tw=Waiting period between two attempts for restoration.

Using this equation, a typical restoration time can be estimated asfollows. Assume that each link has 16 channels of which 12 are used fornormal traffic and 4 channels are available for restoration. Allchannels are restored in three attempts. Because of the simplicity ofthe RS signal, even with a low bit rate such as 10 Mb/s, a processingtime of 0.5 millisecond should be sufficient. If 8 bits are reserved foreach the originating node ID, terminating node ID, and framing, thenwithin a period of 0.5 millisecond, there are 208 frames to detect andconfirm the simple repeated message. Another 0.5 millisecond should besufficient for connecting or disconnecting the channels. Therefore, 1millisecond for tp is a conservative estimate. An estimate of 1millisecond for tw is also conservative. With these numbers, therestoration time for a link failure is conservatively estimated to be 68milliseconds. Out of the 68 milliseconds, the signal propagation time of28 milliseconds is fixed. However, it is possible to reduce theprocessing time. If it is reduced to 0.5 millisecond, for example, thetotal restoration time is less than 50 milliseconds.

The restoration from node failure however, will take longer. It willvary depending on the connectivity of the network. If the degree of thefailed node in a network is four or five, a restoration time of lessthan 500 milliseconds can be achieved.

Although various embodiments are specifically illustrated and describedherein, it will be appreciated that modifications and variations of thepresent invention are covered by the above teachings and within thepurview of the appended claims without departing from the spirit andintended scope of the invention. For example, although the method andapparatus described above uses a link based approach, the method andapparatus can be modified to apply in the case of path based restorationas well and still fall within the scope of the invention. In anotherexample, although the various embodiments of the invention utilizeoptical cross-connects, it can be appreciated that electricalcross-connects fall within the scope of the invention as well.

1. A method comprising: automatically causing information signals froman originating node to a terminating node to be automatically reroutedvia an alternate path in a network, said network comprising a pluralityof nodes, each pair of nodes of said plurality of nodes connected by alink, each link comprising a plurality of information channels and aplurality of restoration channels, said alternate path determined basedupon a restoration signal sent via a restoration channel of saidplurality of restoration channels of a determined link and a first setof idle signals sent via each restoration channel of each link, saidalternate path automatically determined responsive to a detection of afailed link in an original path between said originating node with saidterminating node.
 2. The method of claim 1, further comprising: sendingsaid restoration signal, said restoration signal comprising a nodeidentification number for said originating node.
 3. The method of claim1, further comprising: sending said restoration signal, said restorationsignal comprising a node identification number for said terminatingnode.
 4. The method of claim 1, further comprising: routing saidrestoration signal via alternate links and at least one intermediatenode until said restoration signal reaches said terminating node, andsaid restoration signal comprising a node identification number for saidoriginating node.
 5. The method of claim 1, further comprising: routingsaid restoration signal via alternate links and at least oneintermediate node until said restoration signal reaches said originatingnode, and said restoration signal comprising a node identificationnumber for said terminating node.
 6. The method of claim 2, wherein saiddetermined link is a link of an intermediate node of said alternatepath.
 7. The method of claim 1, further comprising: sending saidrestoration signal over a restoration channel for each link connected toan intermediate node except for a detected restoration channel via whichsaid restoration signal was received by said intermediate node, saidintermediate node a node on said alternate path.
 8. The method of claim1, further comprising: sending an idle signal over said restorationchannel if an NID of said restoration signal matches an NID of areceived second restoration signal, said second restoration signalreceived via a receiving restoration channel at an intermediate node onsaid alternate path.
 9. The method of claim 1, further comprising:sending said restoration signal over said restoration channel if an NIDof said restoration signal does not match an NID of a received secondrestoration signal, said second restoration signal received via areceiving restoration channel at an intermediate node on said alternatepath.
 10. The method of claim 1, further comprising: receiving an idlesignal of said first set of idle signals.
 11. The method of claim 1,further comprising: disconnecting an input adapted to receive saidinformation signals from said failed link.
 12. The method of claim 1,further comprising: connecting an input adapted to receive saidinformation signals to said determined link.
 13. The method of claim 1,further comprising: sending an idle signal via all links connected tosaid terminating node except for a particular link over which saidrestoration signal was received by said terminating node.
 14. The methodof claim 1, further comprising: sending an idle signal via all linksconnected to said originating node except for a particular link overwhich said restoration signal was received by said originating node. 15.The method of claim 1, further comprising: determining said alternatepath via a plurality of activities until at least one terminatingcondition is fulfilled from a group comprising: (1) all failed channelsare restored; (2) there are no more available restoration channels onany link connected to one of said originating node and said terminatingnode; (3) a predetermined delay period expires and said restorationsignal is not received by one of said originating node and saidterminating node; and (4) a node receives a command from a centralcontroller to halt restoration.
 16. The method of claim 1, furthercomprising: detecting a repair of said failed link.
 17. The method ofclaim 1, further comprising: receiving an idle signal at saidoriginating node and said terminating node, said second idle signalindicative of a repair of said failed link.
 18. The method of claim 1,further comprising: routing said information signals from said alternatepath to said original path responsive to a detected repair of saidfailed link.
 19. The method of claim 1, further comprising: sending anidle signal over said restoration channel responsive to a detectedrepair of said failed link.
 20. A method comprising: automaticallydetermining an alternate path from an originating node to a terminatingnode, said alternate path adapted to reroute information signals fromsaid originating node to said terminating node in a network, saidnetwork comprising a plurality of nodes, each pair of nodes of saidplurality of nodes connected by a link, each link comprising a pluralityof information channels and a plurality of restoration channels, saidalternate path determined based upon a restoration signal sent via arestoration channel of said plurality of restoration channels of adetermined link and a first set of idle signals sent via eachrestoration channel of each link, said alternate path determinedresponsive to a detection of a failed link in an original path betweensaid originating node with said terminating node.
 21. A methodcomprising: automatically causing information signals from anoriginating node to a terminating node to be automatically rerouted viaan alternate path in a network, said network comprising a plurality ofnodes, each pair of nodes of said plurality of nodes connected by alink, each link comprising an information channel and a restorationchannel, said alternate path determined for said information signalsbased upon a restoration signal sent via a restoration channel of saidplurality of restoration channels of a determined link and a first setof idle signals sent via each restoration channel of each link, saidalternate path automatically determined responsive to a detection of afailed link in an original path between said originating node with saidterminating node.
 22. The method of claim 21, wherein said alternatepath is determined via an identification of information channels fornodes having links terminating at said failed node.
 23. The method ofclaim 21, further comprising: ranking said plurality of nodes via aconnection map.
 24. The method of claim 21, further comprising:sequentially restoring information channels for nodes having linksterminating at said failed node according to a ranking of nodes untilall said information channels are restored, said ranking of nodesdetermined via a connection map.